Would-be jihadist Sid Ahmed Ghlam is a scholarship student who spent his adolescence shuttling between France and Algeria, drawing the attention of the French security services with social network postings expressing his desire to join Islamist militants in Syria.

He stands accused of planning an attack on a church in the suburbs of Paris after police uncovered detailed plans at his student residence in the capital, along with a stash of weapons and documents full of references to militant groups.

The 24-year-old Algerian was exposed purely by chance on Wednesday after he called an ambulance saying he had been shot in the leg during an armed robbery at his home. But the IT student's online activities had already raised sufficient alarm to warrant his own police file.

Police said DNA evidence linking him to the murder a 32-year-old woman, Aurelie Châtelain, who was found shot dead in her car over the weekend near the capital was also found after investigators combed his apartment and car.

Ghlam moved to France in 2001 with his mother to join his father in the small eastern town of Saint-Dizier, but had to return to Algeria two years later aged 11, lacking the legal documents needed to remain in France.

He would return aged 19 to take up his studies in the northern city of Reims, and later in Paris, where he lived in an ultra-modern student apartment building and attracted little attention from neighbours.

"It's the first time that we've heard talk of him since he took the apartment, there has been nothing unusual. He paid his rent," a representative for the building said.

But his Facebook activity told a very different story. Ghlam, like hundreds of others, used the social network to express "his desire to leave for Syria" and wage jihad, a police source said.

But surveillance by the intelligence services in the last 16 months found nothing solid enough to warrant further investigation, the French government said on Wednesday.

- 'Church target' -

Even though he had no criminal record, the police source said he was implicated in an alleged assault in August 2013, but the victim withdrew their complaint.

Paris prosecutor Francois Molins said Ghlam may have been in touch with someone in Syria who may have suggested to him "to target a church".

Several of his friends and family have since been detained. However, his sister told AFP that her brother was not an extremist. "My brother did not change. He was not radicalised. I am shocked by all that, we do not believe it."

Late on Wednesday, a 25-year-old woman known to Ghlam was taken in for questioning. A Muslim convert of two years standing, the woman "didn't have a boyfriend", her sister told AFP, adding she never spoke about terrorism.

The arrested woman was the only one in her neighbourhood who wore the niqab, the Islamic veil that covers most of the face, sources close to the police inquiry said.

Hundreds of French nationals have joined jihadist ranks in Iraq and Syria, accounting for almost half the European fighters there, according to a report released this month by the upper house Senate.

France has been on high alert since 16 people died in January's jihadi attacks on the Charlie Hebdo satirical magazine and a Jewish supermarket in Paris two days later.